We propose to analyze the shape and conduction velocity of the action potential of Purkinje fibers of the sheep. Measurements will be made of the ionic mechanisms which are the energy source of the action potential and of the resistive and capacitive processes which are the electrical load for the action potential, since both determine the conduction velocity and shape of the action potential. The measurement of the electrical load is a pre-requisite for the design and interpretation of experiments on the ionic mechanism of the action potential and so such measurements will be made first. The load will be characterized by measurements of both longitudinal and transverse impedance, made with sinusoidal, rectangular pulse and stochastic signals. The stochastic signals will be particularly useful since they allow measurement in the shortest time. A voltage clamp system will be built which measures potential with a microelectrode and injects current with a double oil/sucrose gap arrangement. The double gap will essentially eliminate leakage current. Precautions will be taken to minimize and remove lumped series resistance and distributed series resistance will be included in the theoretical analysis of the experimental data. Measurements of equivalent circuit will be made on each preparation and so the values of the series resistance will be known. Longitudinal uniformity of potential will be directly measured. The resulting records of ionic current will be analyzed with models which do not assume the independence of individual ionic currents.